Abrasive, method of polishing target member and process for producing semiconductor device

ABSTRACT

To polish polishing target surfaces of SiO 2  insulating films or the like at a high rate without scratching the surface, the present invention provides an abrasive comprising a slurry comprising a medium and dispersed therein at least one of i) cerium oxide particles constituted of at least two crystallites and having crystal grain boundaries or having a bulk density of not higher than 6.5 g/cm 3  and ii) abrasive grains having pores. Also provided are a method of polishing a target member and a process for producing a semiconductor device which make use of this abrasive.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of application Ser. No.10/042,271, filed Jan. 11, 2002 now U.S. Pat. No. 7,115,021, which is aContinuation Application of application Ser. No. 09/581,814, filed Sep.8, 2000 submitted under 35 U.S.C. §371 on Jun. 19, 2000 now U.S. Pat No.6,343,976. The contents of Ser. No. 09/581,814 are incorporated hereinby reference in their entirety. Ser. No. 09/581,814 is a National StageApplication filed under 35 U.S.C. §371 of International (PCT)Application No. PCT/JP98/05736, filed Dec. 18, 1998.

TECHNICAL FIELD

This invention relates to an abrasive, a method of polishing a targetmember, and a process for producing a semiconductor device.

BACKGROUND ART

Conventionally, in the steps of fabricating semiconductor devices,studies are commonly made on colloidal silica type abrasives used aschemical mechanical abrasives for smoothing inorganic insulating filmlayers such as SiO₂ insulating films formed by processes such asplasma-assisted CVD (chemical vapor deposition) and low-pressure CVD.The colloidal silica type abrasives are produced by growing silicaparticles into grains by a method of, e.g., thermal decomposition oftetrachlorosilicic acid, and making pH adjustment with an alkalisolution containing no alkali metal, such as ammonia. Such abrasives,however, can not provide any sufficient rate of polishing for thepolishing of inorganic insulating films, and have a technical problem oflow polishing rate for their practical utilization.

Meanwhile, cerium oxide abrasives are used as glass surface abrasivesfor photomasks. Cerium oxide particles are useful for finishmirror-polishing because they have a lower hardness than silicaparticles and alumina particles and hence may hardly scratch polishedsurfaces. Also, cerium oxide, which is known as a strong oxidant, haschemically active nature. Making the most of this advantage, itsapplication in chemical mechanical abrasives for the insulating films isuseful. However, when such cerium oxide abrasives for glass surfaceabrasives for photomasks are used in the polishing of inorganicinsulating films as they are, they have so large a primary particlediameter as to scratch, on polishing, the insulating film surface to avisually observable extent.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an abrasive that canpolish polishing target surfaces of SiO₂ insulating films or the like ata high rate without scratching the surface, a method polishing a targetmember, and a process for producing a semiconductor device.

The present invention provides an abrasive comprising a slurrycomprising a medium and dispersed therein cerium oxide particlesconstituted of at least two crystallites and having crystal grainboundaries.

The cerium oxide particles having crystal grain boundaries maypreferably have diameter with a middle value of from 60 nm to 1,500 nm,more preferably from 100 nm to 1,200 nm, and most preferably from 300 nmto 1,000 nm. The crystallites may preferably have diameter with a middlevalue of from 5 nm to 250 nm, and more preferably from 5 nm to 150 nm.Preferably usable are particles wherein the cerium oxide particleshaving crystal grain boundaries have diameter with a middle value offrom 300 nm to 1,000 nm and the crystallites have diameter with a middlevalue of from 10 nm to 50 nm. Also preferably usable are particleswherein the cerium oxide particles having crystal grain boundaries havediameter with a middle value of from 300 nm to 1,000 nm and thecrystallites have diameter with a middle value of from 50 nm to 200 nm.The cerium oxide particles having crystal grain boundaries maypreferably have a maximum diameter not larger than 3,000 nm, and thecrystallites may preferably have a maximum diameter not larger than 600nm. Those in which the crystallites have a diameter of from 10 nm to 600nm are preferred.

The present invention also provides an abrasive comprising a slurrycomprising a medium and dispersed therein abrasive grains having pores.As the abrasive grains, cerium oxide particles may preferably be used.

The pore may preferably be in a porosity of from 10% to 30% asdetermined from the ratio of a density measured with a pycnometer to atheoretical density determined by X-ray Rietvelt analysis. The pores mayalso preferably have a pore volume of from 0.02 cm³/g to 0.05 cm³/g asmeasured by the B. J. H. (Barret, Joyner and Halende) method.

The present invention still also provides an abrasive comprising aslurry comprising a medium and dispersed therein cerium oxide particleshaving a bulk density not higher than 6.5 g/cm³. Those having a bulkdensity of from 5.0 g/cm³ to 5.9 g/cm³ are preferred.

As the medium, water may preferably be used. The slurry may contain adispersant. The dispersant may preferably be at least one selected froma water-soluble organic polymer, a water-soluble anionic surface-activeagent, a water-soluble nonionic surface-active agent and a water-solubleamine. Ammonium polyacrylate may preferably be used.

The present invention further provides an abrasive comprising ceriumoxide particles constituted of at least two crystallites and havingcrystal grain boundaries; cerium oxide particles with a diameter notsmaller than 1 μm occupying at least 0.1% by weight of the total weightof the cerium oxide particles, and the cerium oxide particles havingcrystal grain boundaries being capable of polishing a target memberwhile collapsing at the time of polishing.

The present invention still further provides an abrasive comprisingcerium oxide particles constituted of at least two crystallites andhaving crystal grain boundaries; the cerium oxide particles havingcrystal grain boundaries being capable of polishing a target memberwhile forming new surfaces not coming into contact with any medium atthe time of polishing.

The present invention still further provides an abrasive comprisingcerium oxide particles constituted of at least two crystallites andhaving crystal grain boundaries, wherein;

-   (1) the content of cerium oxide particles having a particle diameter    not smaller than 0.5 μm after polishing, measured by centrifugal    sedimentation after a target member has been polished, is in a ratio    of not more than 0.8 with respect to the content of cerium oxide    particles having a particle diameter not smaller than 0.5 μm before    polishing, measured likewise by centrifugal sedimentation;-   (2) cerium oxide particle diameter at D99% by volume after    polishing, measured by laser diffraction after a target member has    been polished, is in a ratio of from 0.4 to 0.9 with respect to    cerium oxide particle diameter at D99% by volume before polishing,    measured likewise by laser diffraction; and-   (3) cerium oxide particle diameter at D90% by volume after    polishing, measured by laser diffraction after a target member has    been polished, is in a ratio of from 0.7 to 0.95 with respect to    cerium oxide particle diameter at D90% by volume before polishing,    measured likewise by laser diffraction.

The method of polishing a target member according to the presentinvention comprises polishing a target member by the use of the abrasivedescribed above. The target member may preferably have a strength higherthan the grain boundary breaking strength of the cerium oxide particles.The target member may be a semiconductor chip on which a silica film hasbeen formed.

The process for producing a semiconductor device according to thepresent invention comprises the step of polishing a semiconductor chipon which a silica film has been formed, by the use of the abrasivedescribed above.

BEST MODE FOR PRACTICING THE INVENTION

Cerium oxide is commonly obtained by firing a cerium compound such ascarbonate, sulfate or oxalate. SiO₂ insulating films formed byTEOS(tetraethoxysilane)-CVD can be polished at a higher rate as thecerium oxide has larger particle diameter and can have less crystalstrain, i.e., has better crystallizability, but tend to be scratched onpolishing. Accordingly, the cerium oxide particles used in the presentinvention are prepared without making them highly crystallizable somuch. Also, since they are used in polishing for semiconductor chips,alkali metals and halogens may preferably be kept in a content of 1 ppmor less.

The abrasive of the present invention has so high a purity as to contain1 ppm or less each of Na, K, Si, Mg, Ca, Zr, Ti, Ni, Cr and Fe and 10ppm or less of Al.

In the present invention, the cerium oxide particles may be prepared byfiring. However, in order to prepare particles not causative of polishscratches, low-temperature firing is preferred which does not make themhighly crystallizable as far as possible. Since the cerium compoundshave an oxidation temperature of 300° C., they may preferably be firedat a temperature of from 400° C. (inclusive) to 900° C. (inclusive). Itis preferable to fire cerium carbonate at a temperature of from 400° C.(inclusive) to 900° C. (inclusive) for 5 to 300 minutes in an oxidizingatmosphere of oxygen gas or the like.

Cerium oxide formed by baking may be pulverized by dry-processpulverization using a jet mill, a ball mill or the like, or bywet-process pulverization using a beads mill, a ball mill or the like.In cerium oxide particles obtained by pulverizing the fired ceriumoxide, single-crystal particles having a small crystallite size andpulverized particles having not been pulverized to crystallite size arecontained. The pulverized particles differ from agglomerates formed byre-agglomeration of single-crystal particles, and are constituted of atleast two crystallites and have crystal grain boundaries. Where thepolishing is carried out using an abrasive containing such pulverizedparticles having crystal grain boundaries, it is presumed that theparticles are broken by the stress at the time of polishing to bringabout active surfaces, which surfaces are considered to contribute tothe high-rate polishing without scratching the polishing target surfacesof SiO₂ insulating films or the like.

In the present invention, a cerium oxide slurry is obtained bysubjecting to dispersion an aqueous solution containing the cerium oxideparticles produced in the manner described above, or a compositioncomprised of cerium oxide particles collected from this aqueoussolution, water and optionally a dispersant. The cerium oxide particlesmay optionally be classified through a filter. Here are no particularlimitations on the concentration of the cerium oxide particles. In viewof readiness to handle suspensions (abrasive), it may preferably bewithin the range of from 0.5 to 10% by weight.

The dispersant may include, as those containing no metal ions,water-soluble organic polymers such as acrylic acid type polymers andpolyvinyl alcohol, water-soluble anionic surfactants such as ammoniumlauryl sulfate and ammonium polyoxyethylene lauryl ether sulfate,water-soluble nonionic surfactants such as polyoxyethylene lauryl etherand polyethylene glycol monostearate, and water-soluble amines such asmonoethanolamine and diethanolamine. The acrylic acid type polymers mayinclude, e.g., polyacrylic acid and ammonium salts thereof,polymethacrylic acid and ammonium salts thereof, and copolymers ofammonium polyacrylate with alkyl(methyl, ethyl or propyl)acrylates.

Of these, ammonium polyacrylate or a copolymer of ammonium polyacrylatewith methyl acrylate is preferred. When the latter is used, the ammoniumpolyacrylate and the methyl acrylate may preferably be in a molar ratioof ammonium polyacrylate/methyl acrylate of from 10/90 to 90/10.

The acrylic acid type polymers may also have a weight-average molecularweight of from 1,000 to 20,000. Those having a weight-average molecularweight more than 20,000 tend to cause changes with time of particle sizedistribution as a result of re-agglomeration. Those having aweight-average molecular weight less than 1,000 can not, in some cases,well be effective for providing dispersibility and preventingsedimentation.

In view of the dispersibility and prevention of sedimentation ofparticles in the slurry, any of these dispersant may be added in anamount ranging from 0.01 part by weight to 5 parts by weight based on100 parts by weight of the cerium oxide particles. In order to improveits dispersion effect, the dispersant and the particles may preferablybe added simultaneously in a dispersion machine at the time ofdispersion treatment. If the amount is less than 0.01 part by weightbased on 100 parts by weight of the cerium oxide particles, theparticles tend to settle. If the amount is more than 5 parts by weight,the particle size distribution tends to change with time because ofre-agglomeration.

As methods of dispersing these cerium oxide particles in water, they maybe dispersion-treated by means of a conventional stirrer. Besides, ahomogenizer, an ultrasonic dispersion machine or a ball mill or the likemay be used. In order to disperse cerium oxide particles having a sizeof submicron order, a wet-process dispersion machine such as a ballmill, a vibratory ball mill, a planetary ball mill, a media agitatingmill or the like may be used. Also, when the slurry should be madehighly alkaline, an alkali substance containing no metal ions, such asammonia water may be added at the time of dispersion treatment or afterthe treatment.

The cerium oxide abrasive of the present invention may be used in theform of the above slurry as it is. Alternatively, additives such asN,N-diethylethanolamine, N,N-dimethylethanolamine,aminoethylethanolamine, an anionic surfactant and polyvinyl alcohol orthe dispersant described above may appropriately be added in accordancewith the form of use to make up the abrasive.

The cerium oxide particles having crystal grain boundaries, to becontained dispersedly in the slurry of the present invention, maypreferably have diameter with a middle value of from 60 nm to 1,500 nm,and the crystallites may preferably have diameter with a middle value offrom 1 nm to 250 nm.

If the cerium oxide particles having crystal grain boundaries havediameter with a middle value smaller than 60 nm or the crystallites havediameter with a middle value smaller than 1 nm, it tends to be difficultto polish the polishing target surfaces of SiO₂ insulating films and thelike at a high rate. If the cerium oxide particles having crystal grainboundaries have diameter with a middle value larger than 1,500 nm or thecrystallites have diameter with a middle value larger than 250 nm, thepolishing target surfaces of SiO₂ insulating films and the like tend tobe scratched. If the cerium oxide particles having crystal grainboundaries have a maximum diameter larger than 3,000 nm, the polishingtarget surfaces of SiO₂ insulating films and the like tend to bescratched. The cerium oxide particles having crystal grain boundariesmay preferably be in a content of from 5 to 100% by volume of the wholecerium oxide particles. If they are less than 5% by volume, thepolishing target surfaces of SiO₂ insulating films and the like tend tobe scratched.

In the above cerium oxide particles, the crystallites may preferablyhave a maximum diameter not larger than 600 nm, and the crystallites maypreferably have a diameter of from 10 nm to 600 nm. If the crystalliteshave a diameter larger than 600 nm, the polishing target surfaces tendto be scratched. If they have a diameter smaller than 10 nm, thepolishing rate tends to be lower.

In the present invention, the diameters of the crystallites and ceriumoxide particles having crystal grain boundaries are measured byobservation with a scanning electron microscope (e.g., S-900,manufactured by Hitachi Ltd.). The particle diameter of particles isdetermined from the length and breadth of the particle. Morespecifically, the length and breadth of the particle are measured andthe square root of the product of the length and breadth is regarded asparticle diameter. Also, the volume of a sphere that is determined fromthe particle diameter thus determined is regarded as the volume of theparticle.

The middle value is the middle value of volume-based particle sizedistribution and is meant to be a particle diameter at which the valueobtained by adding the volume proportions of particles from among thosehaving smaller particle diameters comes to be 50%. More specifically,when particles in an amount of volume proportion Vi % are present withinthe range of particle diameters in a certain interval Δ and where anaverage particle diameter in the interval Δ is represented by di,particles having particle diameter di are assumed to be present in anamount of Vi % by volume. The di at which the value obtained by addingexistence proportion Vi(% by volume) of particles from among thosehaving smaller particle diameters di comes to be Vi=50% is regarded asthe middle value.

The cerium oxide particles having pores, to be contained dispersedly inthe slurry of the present invention, may preferably have a porosity offrom 10 to 30%. This porosity is determined by calculating it from theratio of a density measured (pure water, 20° C.) with a pycnometer to atheoretical density determined by X-ray Rietvelt analysis. The ceriumoxide particles having pores may preferably have a pore volume of from0.02 to 0.05 cm³/g.

If the cerium oxide particles have a porosity lower than 10% or a porevolume smaller than 0.02 cm³/g, the polishing target surfaces of SiO₂insulating films and the like can be polished at a high rate but tend tobe scratched on polishing. If on the other hand they have a porosityhigher than 30% or a pore volume larger than 0.05 cm³/g, the polishingtarget surfaces of SiO₂ insulating films and the like, though notscratched on polishing, tend to be polished at a low rate.

The present invention also provides an abrasive comprising a slurrycomprising a medium and dispersed therein cerium oxide particles havinga bulk density not higher than 6.5 g/cm³. If the cerium oxide particleshave a bulk density higher than 6.5 g/cm³, the polishing target surfacesof SiO₂ insulating films may be scratched. The cerium oxide particlesmay preferably have a bulk density of from 5.0 to 5.9 g/cm³. If it islower than this lower limit value, the polishing rate may lower. If itis higher than the upper limit value, the polishing target surfaces tendto be scratched. The bulk density referred to in the presentspecification is the density of powder measured with a pycnometer. Inthe measurement, pure water is used as the liquid injected into thepycnometer, and the measurement was made at 20° C.

Primary particles constituting the cerium oxide particles containeddispersedly in the slurry of the present invention may preferably havean aspect ratio of from 1 to 2 and a middle value of 1.3. The aspectratio is measured with a scanning electron microscope (e.g., S-900,manufactured by Hitachi Ltd.).

The slurry of the present-invention may preferably have a pH of from 7to 10, and more preferably from 8 to 9.

The slurry, after adjustment of its pH, may be put in a container madeof polyethylene or the like, and may be used after it has been left at 5to 55° C. for 7 days or more, and preferably 30 days or more, wherebythe polishing target surfaces can be made to be less scratched. Theslurry of the present invention can stand dispersed so well and maysettle so slowly that its rate of change in concentration after leavingfor 2 hours is less than 10% at any height in a columnar container of 10cm diameter and 1 m high.

The present invention further provides an abrasive comprising ceriumoxide particles constituted of at least two crystallites and havingcrystal grain boundaries; cerium oxide particles with a diameter notsmaller than 1 μm occupying at least 0.1% by weight of the total weightof the cerium oxide particles, and the cerium oxide particles havingcrystal grain boundaries being capable of polishing a target memberwhile collapsing at the time of polishing. The cerium oxide particleshaving a diameter not smaller than 1 μm may preferably be in a contentof from 0.1 to 50% by weight, and more preferably from 0.1 to 30% byweight.

The content of the cerium oxide particles having a diameter not smallerthan 1 μm is measured by measuring the intensity of light transmittedthrough particles while being shut out by the particles, using asubmerged-particle counter. As a measuring device, for example Model 770AccuSizer (trade name), manufactured by Particle Sizing System, Inc.,may be used.

The present invention still further provides an abrasive in which, whena target member is polished, the cerium oxide particles having crystalgrain boundaries are capable of polishing the target member whileforming new surfaces not coming into contact with any medium.

The present invention still further provides an abrasive in which thecontent of cerium oxide particles having a particle diameter not smallerthan 0.5 μm after polishing, measured by centrifugal sedimentation aftera target member has been polished, is in a ratio of not less than 0.001with respect to the content of that before polishing. The centrifugalsedimentation is a method of measuring the content of cerium oxideparticles by measuring the intensity of light transmitted throughparticles having been settled by centrifugal force. As a measuringdevice, for example SA-CP4L (trade name), manufactured by shimadzuCorporation, may be used.

The present invention still further provides an abrasive in which ceriumoxide particle diameter at D99% by volume after polishing, measured bylaser diffraction after a target member has been polished, is capable ofbeing in a ratio of from 0.4 to 0.9 with respect to cerium oxideparticle diameter at D99% by volume before polishing.

In the abrasive of the present invention, cerium oxide particle diameterat D90% by volume after polishing, measured by laser diffraction after atarget member has been polished, may also be capable of being in a ratioof from 0.7 to 0.95 with respect to cerium oxide particle diameter atD90% by volume before polishing.

Incidentally, the terms “after a target member has been polished” ismeant to be “after a polishing target surface has been polished by i)setting a target member to a holder to which a substrate-attachingsuction pad for holding a target member to be polished has beenfastened, ii) putting the holder with the polishing target surface sidedown, on a platen to which a polishing pad made of a porous urethaneresin, iii) further putting a weight so as to apply a working load of300 g/cm², and iv) rotating the platen at 30 rpm for 1 hour whiledropping the above abrasive on the platen at a rate of 50 ml/minute.”Here, the abrasive after polishing is circulated to be reused, and 750ml of the abrasive is used in total.

The measurement by laser diffraction may be made using, e.g., MasterSizer Microplus (refractive index: 1.9285; light source: He-Ne laser;absorption: 0), manufactured by Malvern Instruments Ltd.

The D99% and D90% are meant to be particle diameters at which the valuesobtained by adding the volume proportions of particles from among thosehaving smaller particle diameters come to be 99% and 90%, respectively.

The inorganic insulating films on which the cerium oxide abrasive of thepresent invention is applied may include SiO₂ films formed by CVD usingSiH₄ or tetraethoxysilane as a silicon source and oxygen or ozone as anoxygen source.

As the target member, usable are, e.g., substrates such as semiconductorsubstrates standing at the stage where circuit components and aluminumwiring have been formed thereon and semiconductor substrates standing atthe stage where circuit components have been formed thereon, and onwhich an SiO₂ insulating film has been formed (semi-fabricatedsubstrates). Also usable are substrates having an SiO₂ insulating filmformed for the purpose of semiconductor isolation (shallow trenchisolation). SiO₂ insulating films formed on such semiconductorsubstrates are polished with the above abrasive to eliminate theunevenness of the SiO₂ insulating film surfaces to make the surfacessmooth over the whole semi-fabricated semiconductor substrate surfaces.Here, as an apparatus for the polishing, any commonly availablepolishing apparatus may be used which have a platen to which a holderfor holding a semi-fabricated semiconductor substrate and a polishingcloth (pad) have been fastened (fitted with a motor whose number ofrevolutions is variable). As the polishing cloth, commonly availablenonwoven fabric, foamed polyurethane, porous fluorine resin and so forthmay be used without any particular limitations. The polishing cloth mayalso preferably be grooved so that the slurry can stand there. There areno particular limitations on polishing conditions. The platen maypreferably be rotated at a low number of revolutions of 100 or less. Thepressure applied to the semi-fabricated semiconductor substrate maypreferably be not higher than 1 kg/cm² so as not to cause scratchesafter polishing. In the course of polishing, the slurry is continuouslyfed to the polishing cloth by means of a pump or the like. There are noparticular limitations on the feed rate, provided that the surface ofthe polishing cloth may preferably be always covered with the slurry.

The semi-fabricated semiconductor substrate having been thus polishedmay preferably be cleaned well in running water and thereafter set on aspin-dryer to drive off any drops of water adhering to the polishedsemi-fabricated semiconductor substrate, followed by drying. On the SiO₂insulating film thus made flat, second-layer aluminum wirings areformed, and an SiO₂ insulating film is again formed between, and on, thewirings by the above process. Thereafter, the film is polished with theabove cerium oxide abrasive to eliminate the unevenness of theinsulating film surface to make the surface flat over the wholesemi-fabricated semiconductor substrate surface. This step is -repeatedprescribed times to produce a semiconductor having the desired number oflayers.

The cerium oxide abrasive of the present invention may be used to polishnot only the SiO₂ insulating films formed on semiconductor substrates,but also SiO₂ insulating films formed on wiring boards having certainwirings; glass; inorganic insulating films such as silicon nitridefilms; optical glass such as photomasks, lenses and prisms; inorganicconductive films such as ITO (indium tin oxide) films; opticalintegrated circuits, photoswitches or optical waveguides which areconstituted of glass and a crystalline material; ends of optical fibers;optical single crystals such as scintillators; solid-state laser singlecrystals; LED sapphire substrates for blue-color lasers; semiconductorsingle crystals such as SiC, GaP and GaAs; glass substrates for magneticdisks; magnetic heads, and the like.

Thus, in the present invention, the target member embraces semiconductorsubstrates on which SiO₂ insulating films have been formed, wiringboards on which SiO₂ insulating films have been formed; glass; inorganicinsulating films such as silicon nitride films; optical glass such asphotomasks, lenses and prisms; inorganic conductive films such as ITOfilms; optical integrated circuits, photoswitches or optical waveguideswhich are constituted of glass and a crystalline material; ends ofoptical fibers; optical single crystals such as scintillators;solid-state laser single crystals; LED sapphire substrates forblue-color lasers; semiconductor single crystals such as SiC, GaP andGaAs; glass substrates for magnetic disks; magnetic heads, and the like.

EXAMPLE 1

(1) Preparation of Cerium Oxide Particles

a. Preparation of Cerium Oxide Particles A:

2 kg of cerium carbonate hydrate was put in a container made ofplatinum, and was fired in the air at 800° C. for 2 hours to obtainabout 1 kg of yellowish white powder. This powder was phase-determinedby X-ray diffraction and was confirmed to be cerium oxide.

The fired powder obtained had particle diameters of 30 to 100 μm. Thesurfaces of fired-powder particles were observed with a scanningelectron microscope, where grain boundaries of cerium oxide wereobservable. Diameters of cerium oxide crystallites surrounded by grainboundaries were measured to find that the middle value of theirdistribution was 190 nm and the maximum value was 500 nm.

Next, 1 kg of the fired powder obtained was dry-process pulverized bymeans of a jet mill. Particles obtained after the pulverization wereobserved with a scanning electron microscope to find that largepolycrystalline particles of 1 μm to 3 μm and polycrystalline particlesof 0.5 μm to 1 μm were mixedly present in addition to smallsingle-crystal particles having the same size as the crystallitediameter. The polycrystalline particles were not aggregates of thesingle-crystal particles. The cerium oxide particles thus obtained bypulverization are hereinafter called cerium oxide particles A.

b. Preparation of Cerium Oxide Particles B:

2 kg of cerium carbonate hydrate was put in a container made ofplatinum, and was fired in the air at 750° C. for 2 hours to obtainabout 1 kg of yellowish white powder. This powder was phase-determinedby X-ray diffraction and was confirmed to be cerium oxide. The firedpowder obtained had particle diameters of 30 to 100 μm.

The surfaces of fired-powder particles were observed with a scanningelectron microscope, where grain boundaries of cerium oxide wereobservable. Diameters of cerium oxide crystallites surrounded by grainboundaries were measured to find that the middle value of theirdistribution was 141 nm and the maximum value was 400 nm.

Next, 1 kg of the fired powder obtained was dry-process pulverized bymeans of a jet mill. Particles obtained after the pulverization wereobserved with a scanning electron microscope to find that largepolycrystalline particles of 1 μm to 3 μm and polycrystalline particlesof 0.5 μm to 1 μm were mixedly present in addition to smallsingle-crystal particles having the same size as the crystallitediameter. The polycrystalline particles were not aggregates of thesingle-crystal particles. The cerium oxide particles thus obtained bypulverization are hereinafter called cerium oxide particles B.

(2) Preparation of Abrasives

a. Preparation of Abrasives A & B:

1 kg of the cerium oxide particles A or B obtained in the above (1), 23g of an aqueous ammonium polyacrylate solution (40% by weight) and 8,977g of deionized water were mixed, and the mixture was exposed toultrasonic waves for 10 minutes with stirring to disperse the ceriumoxide particles to obtain a slurry.

The slurry thus obtained was filtered with a 1 μm filter, furtherfollowed by addition of deionized water to obtain an abrasive with asolid content of 3% by weight. The abrasive obtained from the ceriumoxide particles A or B is hereinafter called abrasive A or B,respectively. The abrasive A or B thus obtained had a pH of 8.3 or 8.3,respectively.

In order to observe particles in the abrasive with an scanning electronmicroscope, the abrasives were each diluted to have a suitableconcentration, followed by drying. Then, diameters of polycrystallineparticles contained therein were measured to find that, in the case ofthe abrasive A making use of the cerium oxide particles A, the middlevalue was 825 nm and the maximum value was 1,230 nm. In the case of theabrasive B making use of the cerium oxide particles B, the middle valuewas 768 nm and the maximum value was 1,200 nm.

The abrasive A was dried, and the density (bulk density) of theparticles obtained was measured to find that it was 5.78 g/ml. Also, thetheoretical density measured by X-ray Rietvelt analysis was 7.201 g/ml.The porosity was calculated form these values to find that it was 19.8%.With regard to the particles obtained by drying the slurry, their porevolume was measured by the B.J.H. method to find that it was 0.033cm³/g.

Next, in order to examine the dispersibility of particles in theabrasive and the electric charges of dispersed particles, zetapotentials of the abrasives A and B were examined. More specifically,the cerium oxide slurry was put in a measuring cell in which electrodesmade of platinum were attached to both sidewalls facing each other, anda voltage of 10 V was applied to the both electrodes. Upon applicationof the voltage, dispersed particles having electric charges migrate tothe electrode side having a polarity opposite to that of the electriccharges. The rate of this migration was determined to determine the zetapotential of particles. As a result of the measurement of zetapotential, it was ascertained that the dispersed particles of both theabrasives A and B stood negatively charged and had absolute values of asgreat as −50 mV and −63 mV, respectively, showing a good dispersibility.

b. Preparation of Abrasives A′ & B′:

1 kg of the cerium oxide particles A or B, 23 g of an aqueous ammoniumpolyacrylate solution (40% by weight) and 8,977 g of deionized waterwere mixed, and the mixture was exposed to ultrasonic waves for 10minutes with stirring to disperse the cerium oxide particles to obtain aslurry.

The slurry thus obtained was filtered with a 0.8 μm filter, furtherfollowed by addition of deionized water to obtain an abrasive with asolid content of 3% by weight. The abrasive obtained from the ceriumoxide particles A or B is hereinafter called abrasive A′ or B′,respectively. The abrasives A′ or B′ thus obtained had a pH of 8.3 or8.3, respectively.

In order to observe particles in the abrasive with an scanning electronmicroscope, the abrasives A and B were each diluted to have a suitableconcentration. Thereafter, diameters of polycrystalline particlescontained therein were measured to find that, in the case of theabrasive A′ making use of the cerium oxide particles A, the middle valuewas 450 nm and the maximum value was 980 nm. In the case of the abrasiveB′ making use of the cerium oxide particles B, the middle value was 462nm and the maximum value was 1,000 nm.

Next, in order to examine the dispersibility of particles in theabrasive and the electric charges of dispersed-particles, zetapotentials of the abrasives A′ and B′ were examined in the same manneras the case of the above abrasives A and B. As a result, it wasascertained that the dispersed particles of both the abrasives stoodnegatively charged and had absolute values of as great as −53 mV and −63mV, respectively, showing a good dispersibility.

(3) Polishing of Insulating Film

A silicon wafer on which an SiO₂ insulating film was formed byTEOS-plasma-assisted CVD was set being attracted and fixed to asubstrate-attaching suction pad fastened to a holder. This holder, as itheld the silicon wafer, was placed on a platen with the insulating filmside down, to which platen a polishing pad made of porous urethane resinwas fastened, and a weight was further placed so as to apply a workingload of 300 g/cm².

Next, the platen was rotated at 30 rpm for 2 minutes while dropping theabrasive A, B, A′ or B′ (solid content: 3% by weight) prepared in thepresent Example, onto the platen at a rate of 50 ml/minute to polish theinsulating film formed on the silicon wafer surface. After thepolishing, this wafer (with film) was detached from the holder, and thenthoroughly cleaned with water, followed by further cleaning for 20minutes by means of an ultrasonic cleaner. After the cleaning, thiswafer was set on a spin dryer to drive off drops of water, followed bydrying for 10 minutes by means of a 120° C. dryer.

With regard to this wafer having been dried, any change in layerthickness of the SiO₂ insulating film before and after the polishing wasmeasured with a light interference type layer thickness measuringinstrument. As the result, it was found that, when the abrasives A, B,A′ and B′ were used, the insulating films were abraded by 600 nm(polishing rate: 300 nm/minute), 580 nm (polishing rate: 290 nm/minute),590 nm (polishing rate: 295 nm/minute) and 560 nm (polishing rate: 280nm/minute), respectively, and were each in a uniform thickness over thewhole wafer surface whichever abrasive was used. Also, the insulatingfilm surfaces were observed using an optical microscope, where any clearscratches were not seen in all the cases.

Using the abrasive A, the SiO₂ insulating film on the silicon wafersurface was also polished in the same manner as in the above case, andthe particle diameter of the abrasive A after polishing was measuredwith a centrifugal sedimentation type particle size distribution meterto find that the content (% by volume) of particles not smaller than 0.5μm was in a ratio of 0.385 with respect to that value before polishing.Here, the time for which the platen was rotated in the course of thepolishing was set to be 1 hour, and 15 sheets of silicon wafers withfilms were polished while changing them successively. Also, the abrasiveafter polishing was circulated to be reused, and 750 ml of the abrasivewas used in total. The particle diameter of the abrasive A afterpolishing was measured with a laser scattering type particle sizedistribution meter to find that the particle diameters at D99% and D90%were 0.491 and 0.804, respectively, with respect to the values beforepolishing. From these values, the abrasive A has the nature of polishingthe target while collapsing and the nature of polishing it whileproducing new surfaces not coming into contact with any medium.

EXAMPLE 2

(2) Preparation of Cerium Oxide Particles

a. Preparation of Cerium Oxide Particles C:

2 kg of cerium carbonate hydrate was put in a container made ofplatinum, and was fired in the air at 700° C. for 2 hours to obtainabout 1 kg of yellowish white powder. This powder was phase-determinedby X-ray diffraction and was confirmed to be cerium oxide. The firedpowder obtained had particle diameters of 30 to 100 μm. The surfaces offired-powder particles were observed with a scanning electronmicroscope, where grain boundaries of cerium oxide were observable.Diameters of cerium oxide crystallites surrounded by grain boundarieswere measured to find that the middle value of their distribution was 50nm and the maximum value was 100 nm.

Next, 1 kg of the fired powder obtained was dry-process pulverized bymeans of a jet mill. Particles obtained after the pulverization wereobserved with a scanning electron microscope to find that largepolycrystalline particles of 2 μm to 4 μm and polycrystalline particlesof 0.5 μm to 1.2 μm were mixedly present in addition to smallsingle-crystal particles having the same size as the crystallitediameter. The polycrystalline particles were not aggregates of thesingle-crystal particles. The cerium oxide particles thus obtained bypulverization are hereinafter called cerium oxide particles C.

b. Preparation of Cerium Oxide Particles D:

3 kg of cerium carbonate hydrate was put in a container made ofplatinum, and was fired in the air at 700° C. for 2 hours to obtainabout 1.5 kg of yellowish white powder. This powder was phase-determinedby X-ray diffraction and was confirmed to be cerium oxide. The firedpowder obtained had particle diameters of 30 to 100 μm.

The surfaces of fired-powder particles were observed with a scanningelectron microscope, where grain boundaries of cerium oxide wereobservable. Diameters of cerium oxide crystallites surrounded by grainboundaries were measured to find that the middle value of theirdistribution was 30 nm and the maximum value was 80 nm.

Next, 1 kg of the fired powder obtained was dry-process pulverized bymeans of a jet mill. Particles obtained after the pulverization wereobserved with a scanning electron microscope to find that largepolycrystalline particles of 1 μm to 3 μm and polycrystalline particlesof 0.5 μm to 1 μm were mixedly present in addition to smallsingle-crystal particles having the same size as the crystallitediameter. The polycrystalline particles were not aggregates of thesingle-crystal particles. The cerium oxide particles thus obtained bypulverization are hereinafter called cerium oxide particles D.

c. Preparation of Cerium Oxide Particles E:

2 kg of cerium carbonate hydrate was put in a container made ofplatinum, and was fired in the air at 650° C. for 2 hours to obtainabout 1 kg of yellowish white powder. This powder was phase-determinedby X-ray diffraction and was confirmed to be cerium oxide.

The fired powder obtained had particle diameters of 30 to 100 μm. Thesurfaces of fired-powder particles were observed with a scanningelectron microscope, where grain boundaries of cerium oxide wereobservable. Diameters of cerium oxide crystallites surrounded by grainboundaries were measured to find that the middle value of theirdistribution was 15 nm and the maximum value was 60 nm.

Next, 1 kg of the fired powder obtained was dry-process pulverized bymeans of a jet mill. Particles obtained after the pulverization wereobserved with a scanning electron microscope to find that largepolycrystalline particles of 1 μm to 3 μm and polycrystalline particlesof 0.5 μm to 1 μm were mixedly present in addition to smallsingle-crystal particles having the same size as the crystallitediameter. The polycrystalline particles were not aggregates of thesingle-crystal particles. The cerium oxide particles thus obtained bypulverization are hereinafter called cerium oxide particles E.

d. Preparation of Cerium Oxide Particles F:

2 kg of cerium carbonate hydrate was put in a container made ofplatinum, and was fired in the air at 600° C. for 2 hours to obtainabout 1 kg of yellowish white powder. This powder was phase-determinedby X-ray diffraction and was confirmed to be cerium oxide. The firedpowder obtained had particle diameters of 30 to 100 μm.

The surfaces of fired-powder particles were observed with a scanningelectron microscope, where grain boundaries of cerium oxide wereobservable. Diameters of cerium oxide crystallites surrounded by grainboundaries were measured to find that the middle value of theirdistribution was 10 nm and the maximum value was 45 nm.

Next, 1 kg of the fired powder obtained was dry-process pulverized bymeans of a jet mill. Particles obtained after the pulverization wereobserved with a scanning electron microscope to find that largepolycrystalline particles of 1 μm to 3 μm and polycrystalline particlesof 0.5 μm to 1 μm were mixedly present in addition to smallsingle-crystal particles having the same size as the crystallitediameter. The polycrystalline particles were not aggregates of thesingle-crystal particles. The cerium oxide particles thus obtained bypulverization are hereinafter called cerium oxide particles F.

(2) Preparation of Abrasives

a. Preparation of Abrasives C, D, E & F:

1 kg of the cerium oxide particles C, D, E or F obtained in the above(1), 23 g of an aqueous ammonium polyacrylate solution (40% by weight)and 8,977 g of deionized water were mixed, and the mixture was exposedto ultrasonic waves for 10 minutes with stirring to disperse the ceriumoxide particles to obtain a slurry.

The slurry thus obtained was filtered with a 2 μm filter, furtherfollowed by addition of deionized water to obtain an abrasive with asolid content of 3% by weight. The abrasive obtained from the ceriumoxide particles C, D, E or F is hereinafter called abrasive C, D, E orF, respectively. The abrasive C, D, E or F thus obtained had a pH of8.0, 8.1, 8.4 or 8.4, respectively.

In order to observe particles in the abrasive with an scanning electronmicroscope, the abrasives were each diluted to have a suitableconcentration, followed by drying. Then, diameters of polycrystallineparticles contained therein were measured to find that, in the case ofthe abrasive C making use of the cerium oxide particles C, the middlevalue was 882 nm and the maximum value was 1,264 nm. In the case of theabrasive D making use of the cerium oxide particles D, the middle valuewas 800 nm and the maximum value was 1,440 nm. In the case of theabrasive E making use of the cerium oxide particles E, the middle valuewas 831 nm and the maximum value was 1,500 nm. In the case of theabrasive F making use of the cerium oxide particles F, the middle valuewas 840 nm and the maximum value was 1,468 nm.

Next, in order to examine the dispersibility of particles in theabrasive and the electric charges of dispersed particles, zetapotentials of the abrasives C, D, E and F were examined in the samemanner as in Example 1. As a result, it was ascertained that theparticles in all the abrasives stood negatively charged and had absolutevalues of as great as −64 mV, −35 mV, −38 mV and −41 mV, respectively,showing a good dispersibility.

b. Preparation of Abrasives C′, D′, E′ & F′:

1 kg of the cerium oxide particles C, D, E or F obtained in the above(1), 23 g of an aqueous ammonium polyacrylate solution (40% by weight)and 8,977 g of deionized water were mixed, and the mixture was exposedto ultrasonic waves for 10 minutes with stirring to disperse the ceriumoxide particles to obtain a slurry.

The slurry thus obtained was filtered with a 0.8 μm filter, furtherfollowed by addition of deionized water to obtain an abrasive with asolid content of 3% by weight. The abrasive obtained from the ceriumoxide particles C′, D′, E′ or F′ is hereinafter called abrasive C′, D′,E′ or F′, respectively. The abrasive C′, D′, E′ or F′ thus obtained hada pH of 8.0, 8.1, 8.4 or 8.4, respectively.

In order to observe particles in the abrasive with an scanning electronmicroscope, the abrasives C′, D′, E′ and F′were each diluted to have asuitable concentration, followed by drying. Then, diameters ofpolycrystalline particles contained therein were measured to find that,in the case of the abrasive C′ making use of the cerium oxide particlesC, the middle value was 398 nm and the maximum value was 890 nm. In thecase of the abrasive D′ making use of the cerium oxide particles D, themiddle value was 405 nm and the maximum value was 920 nm. In the case ofthe abrasive E′ making use of the cerium oxide particles E, the middlevalue was 415 nm and the maximum value was 990 nm. In the case of theabrasive F′ making use of the cerium oxide particles F, the middle valuewas 450 nm and the maximum value was 1,080 nm.

Next, in order to examine the dispersibility of particles in theabrasive and the electric charges of dispersed particles, zetapotentials of the respective abrasives were examined in the same manneras in Example 1. As a result, it was ascertained that the dispersedparticles of all the abrasives stood negatively charged and had absolutevalues of as great as −58 mV, −55 mV, −44 mV and −40 mV, respectively,showing a good dispersibility.

(3) Polishing of Insulating Film

An SiO₂ insulating film formed on a silicon wafer surface was polished,cleaned and dried and any change in layer thickness of the SiO₂insulating film was measured all in the same manner as in Example 1except for using the abrasive C, D, E, F, C′, D′, E′ or F′ prepared inthe present Example. As the result, it was found that, when theabrasives C, D, E, F, C′, D′, E′ and F′ were used, the insulating filmswere abraded by 740 nm (polishing rate: 370 nm/minute), 730 nm(polishing rate: 365 nm/minute), 750 nm (polishing rate: 375 nm/minute),720 nm (polishing rate: 360 nm/minute), 700 nm (polishing rate: 350nm/minute), 690 nm (polishing rate: 345 nm/minute), 710 nm (polishingrate: 355 nm/minute) and 710 nm (polishing rate: 355 nm/minute),respectively, and were each in a uniform thickness over the whole wafersurface whichever abrasive was used. Also, the insulating film surfaceswere observed using an optical microscope, where any clear scratcheswere not seen in all the cases.

COMPARATIVE EXAMPLE

An SiO₂ insulating film formed on a silicon wafer surface by TEOS-CVDwas polished in the same manner as in Examples 1 and 2 except for usingas an abrasive a slurry prepared by dispersing silica having no pores.This slurry had a pH of 10.3, and contained 12.5% by weight of SiO₂particles. Polishing conditions were the same as those in Examples 1 and2.

The insulating film after polishing was observed, where, although noscratches due to polishing were seen and the surface was uniformlypolished, the insulating film was abradable by only 150 nm (polishingrate: 75 nm/minute) as a result of the polishing for 2 minutes.

POSSIBILITY OF INDUSTRIAL APPLICATION

As described above, the present invention makes it possible to polishpolishing target surfaces of SiO₂ insulating films or the like at a highrate without scratching the surface.

1. A method of polishing a target member, comprising polishing a targetmember by the use of an abrasive comprising a slurry including a mediumand cerium oxide particles dispersed in said medium, said cerium oxideparticles constituted of at least two crystallites and having crystalgrain boundaries, wherein: cerium oxide particles with a diameter notsmaller than 1 μm occupies at least 0.1% by weight of the total weightof the cerium oxide particles; and said cerium oxide particles havingcrystal grain boundaries have a characteristic of polishing a targetmember while collapsing.
 2. The method of polishing a target memberaccording to claim 1, wherein said target member has a strength higherthan the grain boundary breaking strength of the cerium oxide particles.3. The method of polishing a target member according to claim 1, whereinsaid target member is a semiconductor chip on which a silica film hasbeen formed.
 4. A process for producing a semiconductor device,comprising the step of polishing a semiconductor chip on which a silicafilm has been formed, by the use of an abrasive comprising a slurryincluding a medium and cerium oxide particles dispersed in said medium,said cerium oxide particles constituted of at least two crystallites andhaving crystal grain boundaries, wherein: cerium oxide particles with adiameter not smaller than 1 μm occupies at least 0.1% by weight of thetotal weight of the cerium oxide particles; and said cerium oxideparticles having crystal grain boundaries have a characteristic ofpolishing a target member while collapsing.
 5. A method of polishing atarget member, comprising polishing a target member by the use of anabrasive comprising a slurry including a medium and cerium oxideparticles dispersed in said medium, said cerium oxide particlesconstituted of at least two crystallites and having crystal grainboundaries, wherein said cerium oxide particles having crystal grainboundaries have a characteristic of polishing a target member whileforming new surfaces of said cerium oxide particles not coming intocontact with any of said medium.
 6. A process for producing asemiconductor device, comprising the step of polishing a semiconductorchip on which a silica film has been formed, by the use of an abrasivecomprising a slurry including a medium and cerium oxide particlesdispersed in said medium, said cerium oxide particles constituted of atleast two crystallites and having crystal grain boundaries, wherein saidcerium oxide particles having crystal grain boundaries have acharacteristic of polishing a target member while forming new surfacesof said cerium oxide particles not coming into contact with any of saidmedium.
 7. An abrasive comprising a slurry including a medium and ceriumoxide particles dispersed in said medium, said cerium oxide particlesconstituted of at least two crystallites and having crystal grainboundaries, wherein: cerium oxide particles with a diameter not smallerthan 1 μm occupies at least 0.1% by weight of the total weight of thecerium oxide particles; and said cerium oxide particles having crystalgrain boundaries have a characteristic of polishing a target memberwhile collapsing.
 8. The abrasive according to claim 7, wherein saidcerium oxide particles having crystal grain boundaries have diameterswith a middle value of from 60 nm to 1,500 nm.
 9. The abrasive accordingto claim 7, wherein said cerium oxide particles having crystal grainboundaries have diameters with a middle value of from 100 nm to 1,200nm.
 10. The abrasive according to claim 7, wherein said cerium oxideparticles having crystal grain boundaries have diameters with a middlevalue of from 300 nm to 1,000 nm.
 11. The abrasive according to claim10, wherein said crystallites have diameters with a middle value of from10 nm to 50 nm.
 12. The abrasive according to claim 10, wherein saidcrystallites have diameters with a middle value of from 50 nm to 200 nm.13. The abrasive according to claim 7, wherein said crystallites havediameters with a middle value of from 5 nm to 250 nm.
 14. The abrasiveaccording to claim 7, wherein said crystallites have diameters with amiddle value of from 5 nm to 150 nm.
 15. The abrasive according to claim7, wherein said cerium oxide particles having crystal grain boundarieshave a maximum diameter not larger than 3,000 nm.
 16. The abrasiveaccording to claim 7, wherein said crystallites have a maximum diameternot larger than 600 nm.
 17. The abrasive according to claim 7, whereinsaid medium is water.
 18. The abrasive according to claim 7, whereinsaid slurry contains a dispersant.
 19. The abrasive according to claim18, wherein said dispersant is at least one selected from awater-soluble organic polymer, a water-soluble anionic surfactant, awater-soluble nonionic surfactant and a water-soluble amine.
 20. Theabrasive according to claim 19, wherein said dispersant is a polyacrylicacid polymer.
 21. The abrasive according to claim 7, wherein content ofcerium oxide particles having a particle diameter not smaller than 0.5μm after polishing, measured by centrifugal sedimentation after a targetmember has been polished, is in a ratio of not more than 0.8 withrespect to that content before polishing.
 22. The abrasive according toclaim 7, wherein cerium oxide particle diameter at D99% by volumemeasured by laser diffraction after a target member has been polished isin a ratio of from 0.4 to 0.9 with respect to that particle diameterbefore polishing.
 23. The abrasive according to claim 7, wherein ceriumoxide particle diameter at D90% by volume measured by laser diffractionafter a target member has been polished is in a ratio of from 0.7 to0.95 with respect to that particle diameter before polishing.
 24. Theabrasive according to claim 7, wherein said abrasive grains have aporosity of from 10% to 30% as determined from a ratio of a true densitymeasured with a pycnometer to a theoretical density determined by X-rayRietvelt analysis.
 25. The abrasive according to claim 24, wherein saidcerium oxide particles have a bulk density not higher than 6.5 g/cm³.26. The abrasive according to claim 25, wherein said bulk density isfrom 5.0 g/cm³ to 5.9 g/cm³.
 27. The abrasive according to claim 7,wherein said abrasive grains have a pore volume of from 0.02 cm³/g to0.05 cm³/g as measured by the B.J.H. method.
 28. The abrasive accordingto claim 27, wherein said cerium oxide particles have a bulk density nothigher than 6.5 g/cm³.
 29. The abrasive according to claim 28, whereinsaid bulk density is from 5.0 g/cm³ to 5.9 g/cm³.
 30. An abrasivecomprising a slurry including a medium and cerium oxide particlesdispersed in said medium, said cerium oxide particles constituted of atleast two crystallites and having crystal grain boundaries, wherein saidcerium oxide particles having crystal grain boundaries have acharacteristic of polishing a target member while forming new surfacesof said cerium oxide particles not coming into contact with any of saidmedium.